Method of batch falling strand devolatilizer

1. A design method of batch falling strand devolatilizers comprising the steps of: constructing a database that contains data of batch falling strand devolatilizer vs. devolatilization of at least one kind of polymer and the database having information related to polymer viscosity, mass flow rate, number of recycles, stage efficiency and process efficiency; data of the number of recycles, the stage efficiency and the process efficiency in the database is substituted into a mass balance difference equation to get a backmixing parameter; when the backmixing parameter is zero or is approaching zero, a liquid diffusion stage efficiency equation having a film equation or a pool equation is integrated with the mass balance difference equation to get a devolatilization process efficiency equation; wherein the film equation is a stage efficiency times a square root of the viscosity and is equal to a constant C 1 times average exposure time of evaporation while the pool equation is a stage efficiency times a square root of the viscosity and is equal to a constant C 2 times average exposure time of evaporation and plus a constant B; and wherein the devolatilization process efficiency equation is written in this form:

1-[1-(constant C1×average exposure time)×viscosity −1/2 /(1+backmixing parameter)] recycles or

1 −[1−(constant C 2 ×average exposure time+constant B)×viscosity −1/2 /(1+backmixing parameter)] recycles ;

making a theoretical value of the process efficiency from calculation of the devolatilization process efficiency equation approach the value of the process efficiency by means of a theoretical value of the backmixing parameter; and substituting the theoretical value of the backmixing parameter into the devolatilization process efficiency equation for performing volume design of the batch falling strand devolatilizer.

2. The design method as claimed in claim 1 , wherein in the step of constructing a database that contains data of batch falling strand devolatilizer vs. devolatilization of at least one kind of polymer and the database having information related to polymer viscosity, mass flow rate, number of recycles, stage efficiency and process efficiency, wherein the stage efficiency is defined as initial amount of volatiles minus amount of volatiles after devolatilization and then divided by the initial amount of volatiles.

3. The design method as claimed in claim 1 , wherein in the step of constructing a database that contains data of batch falling strand devolatilizer vs. devolatilization of at least one kind of polymer and the database having information related to polymer viscosity, mass flow rate, number of recycles, stage efficiency and process efficiency, wherein the process efficiency is equal to initial concentration of volatiles minus volatile concentration after N times of devolatilization and then divided by the initial concentration of volatiles.

4. The design method as claimed in claim 1 , wherein the data of the recycles, the stage efficiency and the process efficiency in the database is substituted into the mass balance difference equation to get a graph of one plus the backmixing parameter vs. polymer viscosity in the step of getting the backmixing parameter.

5. The design method as claimed in claim 1 , wherein the film equation is obtained by integration of an equation—the stage efficiency times square root of the viscosity equals to a constant A 1 divided by mass flow rate, polymer density and volume of the batch falling strand devolatilizer.

6. The design method as claimed in claim 5 , wherein the equation—the stage efficiency times square root of the viscosity equals to a constant A 1 divided by mass flow rate is obtained by integration of a liquid diffusion equation—E f =λ j k c s v , λ j =a 1 /mass flow rate, k c =2[D AB /(πt)] 1/2 and D AB =7.4*10 −8 (ψ B M B ) 1/2 T/(μV A 0.6 and the E f is the stage efficiency, λ j is evaporation time of the j-th stage of the devolatilization process, k c is stage average mass transfer coefficient, s v is stage area/volume ratio, a 1 is constant of the film, D AB is volatiles diffusion coefficient, π is a constant representing the ratio of the circumference of a circle to its diameter, t is operation time of the film under vacuum, ψ B is association parameter of solvent B, M B is molecular weight, T is absolute temperature, and V A is molar volume of volatiles.

7. The design method as claimed in claim 1 , wherein the pool equation is obtained by integration of a equation—the stage efficiency times square root of the viscosity equals to a constant A 2 divided by mass flow rate and then plus a constant B, polymer density and volume of the batch falling strand devolatilizer.

8. The design method as claimed in claim 7 , wherein the equation—the stage efficiency times square root of the viscosity equals to a constant A 2 divided by mass flow rate and then plus a constant B is obtained by integration of a liquid diffusion equation—E f =λ j k c s v , λ j =a 2 /mass flow rate+b, k c =2[D AB /(πt)] 1/2 and D AB =7.4*10 −8 (ψ B M B ) 1/2 T/(μV A 0.6 ) while E f is the stage efficiency, λ j is evaporation time of the j-th stage of the devolatilization process, k c stage average mass transfer coefficient, s v is stage area/volume ratio, a 2 is constant of the film, b is pool constant, D AB is volatiles diffusion coefficient, π is a constant representing the ratio of the circumference of a circle to its diameter, t is operation time of the film under vacuum, Ψ B is association parameter of solvent B, M B is molecular weight, T is absolute temperature, and V A is molar volume of volatiles.

9. The design method as claimed in claim 1 , wherein data of the polymer viscosity and the mass flow rate in the database is substituted into the film equation and the pool equation and then take either the film equation or the pool equation with higher stage efficiency as the liquid diffusion stage efficiency equation.